Sulfonated fatty acid alkyl ester surfactants (alternatively referred to as .alpha.-sulfo fatty acid alkyl ester surfactants, alkyl ester sulfonate surfactants, etc.) are well-known in the detergent field and have been disclosed in, e.g., U.S. Pat. Nos. 5,118,440 (Cutler et al) and 4,438,025 (Satsuki et al), Japanese Laid Open Patent Publication Number 60-133097 (Application No. Showa 58-240021), Japanese Laid Open Patent Publication Number Sho 63-12466 (Patent Application No. Sho 61-151030), Japanese Laid Open Patent Publication Number Sho 59-105099 (Patent Application No.: Sho 57-215962), Japanese Laid Open Patent Publication Hei 2-173196 (Patent Application No. Sho 63-330479), Japanese Laid Open Patent Publication Number Sho 62-43500 (Patent Application No.: Sho 60-183729), and Japanese Laid Open Patent Publication Number Sho 50-151905 (Patent Application No.: 49-60284). Several processes for the manufacture of these sulfonated fatty acid alkyl ester surfactants have been disclosed in, e.g., U.S. Pat. Nos. 4,695,409 (Piorr et al) and 4,820,451 (Piorr et al), German Patent Application 3 535 184 (Imamura et al), Japanese Laid Open Patent Publication Number 290842/90 (Application Number 113423/89), and "The Journal of the American Oil Chemists Society", Vol. 52 (1975), pp. 323-329.
The processes for making the sulfonated fatty acid alkyl ester surfactants described in the technical literature, though, disclose the practicability and desirability of performing the neutralization step in aqueous media. The art has recognized certain problems inherent to such process steps, particularly handling difficulties and hydrolysis reactions. Certain of the processes yield undesirable levels of impurities such as sulfonated fatty acid salts (disalt), thereby producing a low-purity sulfonated fatty acid alkyl ester surfactant. These impurities deteriorate the desirable cleaning and viscosity characteristics of the sulfonated fatty acid alkyl ester surfactant.
Other processes described in the technical literature have observed the desirability of performing the neutralization of sulfonated fatty acid alkyl ester acid mix in a substantially anhydrous, lower alcohol solvent medium. See, e.g., Japanese Patent OPI Publication 90-290842. Unlike processes performed in aqueous media, processes performed under substantially anhydrous neutralization conditions result in a highly undesirable impurity, dimethyl sulfate (DMS). This impurity can be minimized by over-neutralizing an alkyl ester sulfonate acid mix to a pH of at least 10. However, at this higher pH range alkyl ester sulfonates readily react with water to convert to another highly undesirable impurity, sulfonated fatty acid salts (disalt).
Now, however, a process for making a sulfonated fatty acid alkyl ester surfactant having low levels of undesirable impurities has been discovered. By conducting the neutralization process step in accordance with the invention herein, in substantially anhydrous media with a substantially anhydrous alkoxide solution formed by combining an alkoxide solution containing a low level of water and a sacrificial ester, the resultant product has a lower level of disalt and DMS impurities. The reaction mixtures exhibit good handling and in-process flow properties. Therefore, it is an object of this invention to provide a process for making sulfonated fatty acid alkyl ester surfactants containing minimal amounts of undesirable impurities. It is a further object of this invention to provide high purity sulfonated fatty acid alkyl ester surfactants having good flow properties during processing and which, following a separate color improvement process, are useful in detergent products.